test: lib: add uthread test

Add a thread framework test to the lib tests. Update the API
documentation to use the test as an example.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

doc/api/uthread.rst

@@ -3,5 +3,17 @@
 Uthread API
 ===========
 
+.. kernel-doc:: include/uthread.h
+   :doc: Overview
+
 .. kernel-doc:: include/uthread.h
    :internal:
+
+Example
+-------
+
+Here is an example of how to use this API:
+
+.. literalinclude:: ../../test/lib/uthread.c
+   :language: c
+   :linenos:

test/lib/Makefile

@@ -32,6 +32,7 @@ obj-$(CONFIG_CRC8) += test_crc8.o
 obj-$(CONFIG_UT_LIB_CRYPT) += test_crypt.o
 obj-$(CONFIG_UT_TIME) += time.o
 obj-$(CONFIG_$(PHASE_)UT_UNICODE) += unicode.o
+obj-$(CONFIG_UTHREAD) += uthread.o
 obj-$(CONFIG_LIB_UUID) += uuid.o
 else
 obj-$(CONFIG_SANDBOX) += kconfig_spl.o

test/lib/uthread.c

@@ -0,0 +1,80 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright 2025 Linaro Limited
+ *
+ * Unit test for uthread
+ */
+
+#include <stdbool.h>
+#include <test/lib.h>
+#include <test/ut.h>
+#include <uthread.h>
+
+static int count;
+
+/* A thread entry point */
+static void worker(void *arg)
+{
+	int loops = (int)(unsigned long)arg;
+	int i;
+
+	for (i = 0; i < loops; i++) {
+		count++;
+		uthread_schedule();
+	}
+}
+
+/*
+ * uthread() - testing the uthread API
+ *
+ * This function creates two threads with the same entry point. The first one
+ * receives 5 as an argument, the second one receives 10. The number indicates
+ * the number of time the worker thread should loop on uthread_schedule()
+ * before returning. The workers increment a global counter each time they loop.
+ * As a result the main thread knows how many times it should call
+ * uthread_schedule() to let the two threads proceed, and it also knows which
+ * value the counter should have at any moment.
+ */
+static int uthread(struct unit_test_state *uts)
+{
+	int i;
+	int id1, id2;
+
+	count = 0;
+	id1 = uthread_grp_new_id();
+	ut_assert(id1 != 0);
+	id2 = uthread_grp_new_id();
+	ut_assert(id2 != 0);
+	ut_assert(id1 != id2);
+	ut_assertok(uthread_create(NULL, worker, (void *)5, 0, id1));
+	ut_assertok(uthread_create(NULL, worker, (void *)10, 0, 0));
+	/*
+	 * The first call is expected to schedule the first worker, which will
+	 * schedule the second one, which will schedule back to the main thread
+	 * (here). Therefore count should be 2.
+	 */
+	ut_assert(uthread_schedule());
+	ut_asserteq(2, count);
+	ut_assert(!uthread_grp_done(id1));
+	/* Four more calls should bring the count to 10 */
+	for (i = 0; i < 4; i++) {
+		ut_assert(!uthread_grp_done(id1));
+		ut_assert(uthread_schedule());
+	}
+	ut_asserteq(10, count);
+	/* This one allows the first worker to exit */
+	ut_assert(uthread_schedule());
+	/* At this point there should be no runnable thread in group 'id1' */
+	ut_assert(uthread_grp_done(id1));
+	/* Five more calls for the second worker to finish incrementing  */
+	for (i = 0; i < 5; i++)
+		ut_assert(uthread_schedule());
+	ut_asserteq(15, count);
+	/* Plus one call to let the second worker return from its entry point */
+	ut_assert(uthread_schedule());
+	/* Now both tasks should be done, schedule should return false */
+	ut_assert(!uthread_schedule());
+
+	return 0;
+}
+LIB_TEST(uthread, 0);